Error with digit_stack

Hello! My code works perfect locally, but checkio verification does not accept it.

From: http://www.checkio.org/mission/digit-stack/solve/

HTTP_USER_AGENT:

Mozilla/5.0 (Windows NT 6.3; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/44.0.2403.107 Safari/537.36


def digit_stack(commands):
    sequence = []
    summ = 0
    for item in commands:
        if item[:3] == 'PUS':
            sequence.append(int(item[5:]))
        elif item[:3] == 'POP':
            if sequence != []:
                summ = summ + int(sequence[-1])
                del sequence[-1]
        else:
            summ += int(sequence[-1])
    return summ